The present invention relates to a Voltage Source Converter having at least one phase leg connecting to opposite poles of a direct voltage side of the converter and comprising a series connection of switching elements, each said switching element having on one hand at least two semiconductor chips having each a semiconductor device of turn-off type and a free-wheeling diode connected in parallel therewith and on the other at least one energy storing capacitor, a mid point of said series connection forming a phase output being configured to be connected to an alternating voltage side of the converter and to divide the phase leg into an upper valve branch and a lower valve branch, each said switching element being configured to obtain two switching states by control of said semiconductor devices of each switching element, namely a first switching state and a second switching state, in which the voltage across said at least one energy storing capacitor and a zero voltage, respectively, is applied across the terminals of the switching element, for obtaining a determined alternating voltage on said phase output.
Such converters with any number of said phase legs are comprised, but they have normally three such phase legs for having a three phase alternating voltage on the alternating voltage side thereof.
A Voltage Source Converter of this type may be used in all kinds of situations, in which direct voltage is to be converted into alternating voltage or conversely, in which examples of such uses are in stations of HVDC-plants (High Voltage Direct Current), in which direct voltage is normally converted into a three-phase alternating voltage or conversely, or in so-called back-to-back-stations in which alternating voltage is firstly converted into direct voltage and this is then converted into alternating voltage. It may also be used to absorb or inject reactive power in the alternating voltage network.
A Voltage Source Converter of this type is known through for example DE 101 03 031 A1 and WO 2007/023064 A1 and is normally called a multi-cell converter or M2LC. Reference is made to these publications for the functioning of a converter of this type. Said switching elements of the converter may have other appearances than those shown in said publications, and it is for instance possible that each switching element has more than one said energy storing capacitor, as long as it is possible to control the switching element to be switched between the two states mentioned in the introduction.
The present invention is primarily, but not exclusively, directed to such Voltage Source Converters configured to transmit high powers, and the case of transmitting high powers will for this reason mainly be discussed hereafter for illuminating but not in any way restricting the invention thereto. When such a Voltage Source Converter is used to transmit high powers this also means that high voltages are handled, and the voltage of the direct voltage side of the converter is determined by the voltages across said energy storing capacitors of the switching elements and is normally set to be half the sum of these voltages. This means that a comparatively high number of such switching elements are to be connected in series or a high number of semiconductor devices, i.e. said semiconductor chips, are to be connected in series in each said switching element, and a Voltage Source Converter of this type is particularly interesting when the number of the switching elements in said phase leg is comparatively high, such as at least 8. A high number of such switching elements connected in series means that it will be possible to control these switching elements to change between said first and second switching state and already at said phase output obtain an alternating voltage being very close to a sinusoidal voltage. This may be obtained already by means of substantially lower switching frequencies than typically used in known Voltage Source Converters of the type shown in FIG. 1 in DE 101 03 031 A1 having switching elements with at least one semiconductor device of turn-off type and at least one free wheeling diode connected in anti-parallel therewith. This makes it possible to obtain substantially low losses and also considerably reduces problems of filtering and harmonic currents and radio interference, so that equipment therefor may be less costly.
However, the great number of switching elements connected in series and the energy storing capacitors belonging to these switching elements makes Voltage Source Converters of this type rather voluminous, so that for instance in the case of a station of a HVDC-plant very large valve halls are to be built for such converters.